Lewis was a playwright, poet and one of the co-founders of Plaid Cymru and a speech he made on the future of the language led to the setting up of Cymdeithas yr Iaith (Welsh language society).

Dr Daniel G Williams, from the Richard Burton Centre for the Study of Wales, which is organising the event, said the arson attack had been carried out against the background of the Ministry of Defence wanting to build a much-opposed facility.

SAUNDERS LEWIS FACTFILE

Poet, novelist, dramatist, critic, politician and one of the founders of Plaid Cymru, Saunders Lewis is one of Wales' leading literary and political icons, and has been dubbed a nationalist hero.

He was born John Saunders Lewis, into a Welsh-speaking family in Wallasey in 1893, and grew up among the Welsh community in Merseyside.

In 1962 Lewis gave a lecture on BBC radio entitled Tynged Yr Iaith (The Fate of the Language).

In this speech Lewis predicted the extinction of the Welsh language and declared that the language would die unless revolutionary methods were used to defend it.

A result of the lecture led to the foundation of Cymdeithas yr Iaith Gymraeg (the Welsh Language Society).

Saunders Lewis died in September 1985 at the age of 91.

"Saunders Lewis was a playwright and he was profoundly aware of the symbolic significance of 1936 being the 400th anniversary of the Act of Union (annexing Wales into England), and was in a sense looking for a cause," said Dr Williams.

During his court case in Caernarfon, Lewis said the "rights of a nation and its traditions were being ignored".

The three men had gone to the authorities to say they were responsible for the arson attack but a jury in Caernarfon failed to agree on a verdict, and another trial was held at the Old Bailey in London.

It was during this time that he was sacked from his job as lecturer at Swansea and there were protests across Wales.

The 'Penyberth Three' were then jailed for nine months at Wormwood Scrubs for the arson attack.

When the three men came back to Wales they were welcomed home by thousands, but it did not last, said Dr Williams.

"One of the debates will be 'what is the legacy - if at all - of Penyberth'.

"The Second World War broke out soon afterwards of course and it was perhaps forgotten or it's effects are still with us - that's a question for historians and cultural critics," he added.

The seminar will be held on Monday from 16:00 to 18:00 GMT in the Keir Hardie Building, Swansea University.